The present invention relates to methods and compositions of suspending proppant in high salinity, low viscosity subterranean treatment fluids.
Subterranean wells (such as hydrocarbon producing wells and water wells) are often stimulated by hydraulic fracturing treatments. In hydraulic fracturing treatments, a viscous fracturing fluid, which may also function as a carrier fluid to carry particulates, is pumped into a portion of a subterranean formation at a rate and pressure such that the subterranean formation breaks down and one or more fractures are formed. Typically, particulates, such as graded sand, are suspended in a portion of the fracturing fluid or another fluid and then deposited into the fractures. These particulates, referred to herein as “proppant particulates” or simply “proppant,” serve to prevent the fractures from fully closing once the hydraulic pressure is removed. By keeping the fracture from fully closing, the proppant aids in forming conductive paths through which fluids may flow.
Fracturing fluids are typically aqueous based fluids, which are viscosified using gelling agents (e.g., polymers) or gelling agents in combination with crosslinking agents. Often, however, premature settling of the proppant particulates may result in complete or partial fracture closure after hydraulic pressure is removed, thereby reducing the production potential of the formation. Additionally, if a tip screen-out operation is performed, premature settling of proppant particulates may result in decreased or completed loss of fluid pumpability due to pressure increases. Also, fluid loss from the fracturing fluid into undesirable locations within the subterranean formation may affect the production potential of the formation and/or the fluid itself. Fluid loss may be controlled by the gelling agents and/or crosslinking agents used to viscosify the fluid or by adding particulates to the fluid for the purpose of closing pore throats or other avenues for fluid loss.
Thus, fracturing fluid design can be very complex, and fracturing fluids are often designed for narrow sets of reservoir and/or pumping constraints. The complexity is further exacerbated because the availability of vast quantities of fresh water for subterranean formation operations has recently become substantially reduced both for environmental and political reasons. As a result, fracturing operations are preferably performed with sources of water such as seawater, produced water from a formation, or reclaimed water. These water sources, however, do not possess the predictable chemistry that fresh water possesses and typically contain high salt concentrations of diverse ions, often in a concentration of greater than 0.5%, greater than 3%, or even greater than 20% to saturation. This “high salt concentration water” or “high salt concentration base fluid” has historically been less effective as a stable fracturing fluid capable of relatively uniformly suspending proppant particulates for delivery into a fracture, largely because gelling agents and/or crosslinking agents may be salted out of the high salt concentration base fluid due to interaction with the anionic polymeric chain of the gelling agent and/or crosslinking agent or due to syneresis. Indeed, the ability of these gelling agents and/or crosslinking agents to viscosify fluids is significantly reduced as salt concentration increases and, in general, fluids with poor viscosity are considered poor at proppant suspension. Therefore, an effective fracturing fluid having a high salt concentration base fluid that may be used in subterranean formation operations may be of benefit to one of ordinary skill in the art.